Physical associations between cyclins, viral oncogenes, and tumor suppressor genes imply a central role for cyclins in growth control. Cyclin D1 was identified as a candidate oncogene in tumor-specific DNA rearrangements. By virtue of DNA amplification, it contributes to 15% of breast cancers. Cyclin D1 protein is overexpressed in an additional 15-20% of breast cancers through unknown mechanisms. To evaluate its oncogenic potential, we overexpressed cyclin D1 in mammary cells in transgenic mice. Overexpression resulted in abnormal mammary proliferation and adenocarcinomas. We propose further studies of cyclin D1's role in breast cancer: AIM 1: Characterization of mechanisms regulating cyclin D1. Cyclin D1 protein is specifically increased in one-third of mammary carcinomas without accompanying increases in mRNA. We will evaluate mechanisms by which this increase in cyclin D1 protein might be regulated using tissue culture models, and we will identify factors regulating cyclin D1 during mammary growth and involution in vivo. AIM 2: Characterization of growth abnormalities in mammary epithelium overexpressing cyclin D1. The functions of cyclin D1 may be more complex than anticipated in vivo since our new preliminary data suggest that cyclin D1 may regulate apoptosis during mammary involution. We will further characterize this new phenotype. We will evaluate interactions between the genes involved in cyclin/cdk 4 complexes that might alter the role of cyclin D1 during mammary involution. By quantifying histologic changes, we will assess the overall function of cyclin D1 in mammary growth and involution. AIM 3: Assessment of second oncogenic events in adenocarcinomas in MMTV-cyclin D1 transcienic mice. Given the prolonged latency period of tumor development in cyclin D1 transgenic mice, the role of "second hits" will be tested by random insertional mutagenesis using mouse mammary tumor virus infections and using differential display of mRNAs expressed at different levels during malignant transformation. Characterization of MMTV insertion sites by inverse PCR and identification of differentially expressed genes during cyclin D1-driven oncogenesis will be used to identify and clone oncogenes that may collaborate with cyclin D1 to cause breast cancer.